Steel substrate for glassing

ABSTRACT

A low carbon glassing steel structure having a very fine grain structure used in glass coating applications without deterioration in strength and impact resistance after being subjected to multiple high temperature thermal cycles, the steel structure having a minimum Charpy V-Notch energy of 50 ft-lbs at -50° F. in both the longitudinal and transverse directions and a minimum tensile strength of 60 KSI, an ASTM ferrite grain size of eight or finer, and the following composition: 
     
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     ELEMENT        PERCENTAGE BY WEIGHT                                       
______________________________________                                    
Carbon         up to 0.10 percent                                         
Manganese      0.45-1.25 percent                                          
Silicon        0.20-0.65 percent                                          
Niobium        0.02-0.06 percent                                          
Iron and incidental                                                       
               Balance                                                    
 Impurities                                                               
______________________________________

RELATED APPLICATIONS

This is a continuation-in-part of our application Ser. No. 817,443 filedJuly 2, 1977, abandoned.

BACKGROUND OF THE INVENTION

The invention relates to a steel substrate structure which receivesglass coatings and, in particular, it relates to a substrate steel platecompatible with glass or ceramic coating methods utilizing elevatedtemperature processing or firing cycles. The term "glassed steel" refersto a composite material with a substrate structure which is at least3/16" in thickness and a glass or ceramic coating approximately 0.035 to0.075" thick.

Glassed steel structures, which are generally constructed with shapedsteel plates as substrates, are frequently used in process vessels forthe chemical, petrochemical, pharmaceutical and food processingindustries. Coatings prepared from silicate frits, with or withoutadditives, have the greatest industrial usage. Variations in compositionof silicate frits are virtually unlimited; frits range fromalkali-alumina borosilicate glasses, which are relatively soft(low-melting) and highly fluxed, to barium crown glasses. Crystallizedceramic coatings wherein the crystallization of the glass is controlledby formulation and heat treatment and by the presence of nucleatingagents which are added to the glass during melting are also known.

Chemical compatibility of the glass coating with the substrate steel isimportant and further the coating must also be physically compatiblewith the underlying metal structure so that undesirable mechanicalstresses are not induced in either material during thermal processing.

The best combination of glass (or ceramic) and metal is obtained whenthe glass has a slightly lower coefficient of thermal expansion than themetal substrate. This "controlled mismatch" results in the glass coatingbeing in a state of compression after cooling which is important sincethese coatings are much stronger in compression than in tension.

Coating processes include heat treatment to promote bonding and sealing.While cooling from such heat treatment, it is important than anunacceptable mismatch not occur in the expansion of the substraterelative to that which the coating will endure during phasetransformation of the substrate. Therefore, there should be no lowertemperature transformation products such as bainite or martensite formedin the steel substrate. If these products occur, cracking or crazing ofthe glass coating is likely due to severe expansion of the steelsubstrate at temperatures where the glass coating is not sufficientlyplastic to endure the strain.

Coatings are usually applied by wet spray, hot dusting, or dippingmethods. Before application by any method, the substrate metal surfaceshould be clean and rough. Abrasive blasting is normally utilized toprovide a satisfactory surface condition. Other surface preparationsteps may be involved. Finally, in the application, the substrate metalreceives a number of coatings with firing after each coating attemperatures up to about 1700° F. Thus, for a quality product, it isessential that the steel substrate have physical characteristicswhereby: (1) it will undergo the various steps utilized in the glassingprocess; (2) it does not promote cracking or crazing of the glasscoating; (3) it provides an adequate structural support for the glasscoating considering the ultimate use of the product; and (4) it does notdeteriorate with the various heat treatments and other steps necessaryfor applying the glass to the steel substrate.

It is known to provide steel substrates for glassing processes which arerelatively free of carbon, that is a low carbon steel plate or a steelplate in which the carbon has been stabilized by the addition oftitanium such as ASTM A562 steel. ASME Code-approved carbon steels whichare currently being used for glassing applications are known by thefollowing ASME numbers: SA285 Gr. B, SA515 Gr.65, and SA516 Gr. 70.However, substrate structures composed of these steels are somewhatlimited as to their ability to be successfully glassed without defectsin the coating. Therefore, a need has existed for a better substratesteel which is more "glassable" than existing steels for both currentlyused and advanced coatings. In addition, improved mechanical propertiesof plate form the steel are desired, particularly notch toughness in therange of -20 to -50° F., in both the longitudinal and transversedirections, such quality to exist after a plurality of heat treatingcycles necessary to fuse the glass and bond it to the metal.

The following publications will aid in an understanding of thebackground of the invention:

Smith, Robert E., "Ceramic-Metal Composites," Chemical Engineering, May10, 1965, pp. 194-200.

Dormer, George J., Norton, George R., and Payne, Burton S., "GlassingCharacteristics of Low-Alloy Steels," Journal of the American CeramicSociety, Vol. 44, No. 8, Aug. 1961 pp. 375-381.

Payne, Jr., B.S., "Nucerite--A New Composite," Chemical EngineeringProgress, Vol. 64, No. 2, February, 1968, pp. 40-43.

SUMMARY OF THE INVENTION

The invention is a new steel substrate structure exclusively for glassand ceramic coating applications, the composition of the substrateproviding excellent glassing characteristics known as glassability. Suchsteel substrates have been successfully tested using both current andadvanced glass coating techniques wherein a number of heat treatmentsand coatings are applied in series. Further, after the required thermalglassing cycles have been completed the mechanical properties of the newsteel substrate--particularly its toughness--remain unimpaired and areadvantageous for ultimate uses to which the glassed steel structure maybe subjected. The new steel substrate has low carbon content (0.10percent maximum), a relatively high silicon level for glassing steels(0.20-0.65 percent) and a niobium addition (0.02-0.06 percent) toproduce both good glassability and desirable mechanical properties inthe steel substrate. The niobium addition causes the formation ofprecipitates in the steel substrate which insures a fine grain size andresults in good mechanical properties after glassing. Silicon andmanganese levels provide a relatively high transformation temperaturerange in the substrate to protect the glass coating from being crazed orcracked. The substrate is composed of a killed steel produced to finegrain practice. The resulting ASTM ferrite grain size number is abouteight and finer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The steel substrate structure of the instant invention is exclusivelyfor glass or ceramic coating applications and, more particularly, forglass coating applications wherein the glass or ceramic is applied in aseries of coatings and undergoes a number of thermal cycles which heatthe steel-glass system up to about 1700° F. The glass is preferablyapplied to the steel substrate, after its surface has been appropriatelyprepared, by wet spraying or hot dusting whereby a number of thin glasscoatings one on top of the other are applied.

Although the finished product may be used for a variety of applications,the primary uses are in process vessels for the chemical industry wherecorrosion is a problem and for the food and pharmaceutical industrieswhere product purity is a concern. The product may be subjected to lowtemperatures and required to support substantial weight in use. Thus,the strength and notch-toughnes of the product at low temperatures arean important factors to be considered in addition to those incident to asuccessful glassing operation as outlined above.

Prior experience has taught that a severe compromise must be madebetween strength and glassability of a steel substrate structure in themanufacture of glasses steel equipment. Low carbon steel structuresavailable for this purpose have had good glassability but low strength.Higher strength steel structures with more carbon have exhibited bothundesirable carbon reaction and hydrogen defects in te glass coating.Steel structures with high levels of titanium added to tie up carbonhave had good glassability and freedom from hydrogen induced defects,but have been relatively expensive. From this background it isconsidered that a preferred substrate steel is one which has relativelylow carbon, which does not undergo adverse phase transformations belowthe temperature at which the glass has hardened, and which will not haveexcessive grain growth after repeated firings thereby exhibiting lowtensile and impact strength. Further, these virtues must exist in aneconomical carbon steel structure.

It occurred to the inventors that a solution to this problem might bepresented by a substrate steel produced to fine grain practice havingthe following composition:

    ______________________________________                                        ELEMENT       PERCENTAGE BY WEIGHT                                            ______________________________________                                        Carbon        up to 0.10 percent                                              Manganese     0.45 to 1.25 percent                                            Silicon       0.20-0.65 percent                                               Niobium       0.02-0.06 percent                                               Phosphorus    0.035 percent maximum                                           Sulfur        0.040 percent maximum                                           Iron          Balance                                                         ______________________________________                                    

Substrates from several heats of a killed steel produced to fine grainpractice within the above composition limits were tested. It was foundthat the transformation temperature range was sufficiently high thatwith the glass-metal system at such temperatures the glass was plastic.A highly sensitive glass coating produced on such steel substrates werewithout significant cracking or crazing and the mechanical properties ofthe steel substrates wer not adversely affected by repeated thermalcycles utilized in the glassing process. The steel substrates of theproducts remained of a fine grain structure and displayed improvedtoughness in both the transverse and longitudinal modes.

The chemistry and mechanical properties after glassing are presented inTables I and II respectively for material prepared from 150 lb.laboratory heats of the new steel substrate compositions. It will benoted that high strength and unusual low-temperature toughness wereobtained, the fine ferrite grain size being maintained through thethermal glassing cycles.

                  TABLE I                                                         ______________________________________                                        Chemistry of Laboratory Heats                                                 Heat  C       Mn      S     P     Si    Al    Nb                              ______________________________________                                        B268  0.08    0.55    0.020 0.011 0.58  0.032 0.038                           B269  0.08    1.07    0.021 0.013 0.32  0.038 0.038                           B278  0.07    0.61    0.017 0.020 0.61  0.058 0.039                           ______________________________________                                    

                                      TABLE II                                    __________________________________________________________________________    Mechanical Properties after Thermal Glassing Cycles                                     Ultimate                                                                  Yield                                                                             Tensile                                                                            Charpy V-Notch Energy                                                Point                                                                             Strength                                                                           at -50° F. (Longitudinal)                                                           Gage      ASTM Ferrite                            Heat  (KSI)                                                                             (KSI)                                                                              (ft-lbs)     (In)      Grain Size #                            __________________________________________________________________________    B268                                                                          Plate 48.4                                                                              62.5 258          3/4       8                                       B269                                                                          Plate 45.1                                                                              64.1 242          3/4       9                                       B278                                                                          Forged Bar                                                                          41.6                                                                              63.1 262          3/4 × 3/4 cross sect.                                                             8                                       Forged Bar                                                                          41.6                                                                              63.0 144 (circumferential)                                                                      2-12 × 3 cross sect.                                                              8                                       __________________________________________________________________________

The continuity of the coating was complete and no evidence of oxidationof the substrate metal was observed. Cracks, pits and similardiscontinuities in the coated surface were absent. The bond strength ofthe glass to the substrate was wholly satisfactory. In summation, anexcellent commercially acceptable glass coating was obtained with thesteel substrate of the invention using advanced glass coatingtechniques. Further, the low temperature notch toughness of the steelwas much superior to currently used steels for the same purposefollowing the thermal glassing cycles.

Subsequent tests performed on production substrate steels after glassinglargely confirmed the physical characteristics obtained in the abovelaboratory test results. Tables III and IV relate to substrate steelswhich were rolled into plate and glass coated. Minimum yield and tensilestrengths of the steel substrates after glassing were found to be 30 ksiand 60 ksi respectively. Thus, the yield strength in the substrates ofthe glassed-steel products was somewhat lower, but not unacceptably so,than that obtained in the laboratory specimens. However, it was clearthat a minimum Charpy V-Notch energy of fifty foot-pounds at -50° F. wasobtainable in both the longitudinal and transverse directions of thesubstrate plate. This is an important and unusual characteristicinasmuch a the notch energy in the transverse direction of the plateusually reflects a reduced toughness relative to the longitudinaldirection.

Although we have described the preferred embodiments of our invention,it should be understood that it is capable of other adaptations andmodifications whereby the claims should be construed not only to coverthe material described in the specification but also equivalentsthereof.

                  TABLE III                                                       ______________________________________                                        Heat  C      Mn      S     P     Si    Al    Nb                               ______________________________________                                        B0068 0.09   0.62    0.018 0.007 0.48  0.060 0.043                            B0137 0.06   0.57    0.020 0.007 0.41  0.033 0.03                             23298-1                                                                             0.08   0.75    0.005 0.013 0.54  0.036 0.049                            ______________________________________                                    

                                      TABLE IV                                    __________________________________________________________________________    Mechanical Properties after Thermal Glassing Cycles                                     Ultimate                                                            Yield     Tensile                                                                             Charpy V-Notch Energy (ft-lbs)                                Point     Strength                                                                            -50° F.                                                                       -75° F.                                                                        -110° F.                                Heat (ksi)                                                                              (ksi) L   T  L   T   L   T  Gage (inch)                             __________________________________________________________________________    B0068                                                                         Plate                                                                              39.2 63.2  69  61 48  30   7   4 1                                       B0137                                                                         Plate                                                                              38.7 60.0  257 75 260 28  19  10 1                                       23298-1                                                                       Plate                                                                              31.6 63.4  73  90 36  109 21  10 11/16                                   __________________________________________________________________________

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent of the United States is:
 1. A low carbonglassing steel structure having a very fine grain structure for use inglass coating applications without deterioration in strength and impactafter multiple high temperature thermal cycles, said steel structurecomposed of:

    ______________________________________                                        ELEMENT        PERCENTAGE BY WEIGHT                                           ______________________________________                                        Carbon         up to 0.10 percent                                             Manganese      0.45-1.25 percent                                              Silicon        0.20-0.65 percent                                              Niobium        0.02-0.06 percent                                              Iron and incidental                                                            impurities    Balance;                                                       ______________________________________                                    

said steel structure having an ASTM ferrite grain size of about 8 orfiner, a yield point in excess of 30000 psi and an ultimate tensilestrength in excess of 60000 psi, and a minimum Charpy V-Notch energy of50 foot-pounds at -50° F. in both the longitudinal and transversedirections of the structure, and after being subjected to said thermalcycles.
 2. A low carbon steel structure in accordance with claim 1 witha glass coating thereon wherein said glass is plastic in thetransformation temperature range of said steel.
 3. A low carbon steel inaccordance with claim 1 which contains a maximum of 0.035 percentphosphorus and 0.040 percent sulfur.
 4. A low carbon steel structure inaccordance with claim 1 which contains a maximum of about 0.015 percentphosphorus and about 0.015 percent sulfur.
 5. A low carbon steel platein accordance with claim 1 which is utilized in a process wherein glasscoatings are provided thereon with little or no deterioration in impactresistance of said plate as a result of the process, said processincluding thermal cycling heat treatments of the steel plate with glassapplied thereto up to about 1700° F.
 6. A low carbon glassing steelsubstrate plate for use in glass coating applications involving multiplethermal cycles at temperatures up to 1700° F. with little or nodeterioration in strength and impact resistance resulting therefrom,said steel substrate having the following composition:

    ______________________________________                                        ELEMENT         PERCENTAGE BY WEIGHT                                          ______________________________________                                        Carbon          up to 0.10                                                    Manganese       0.45-0.80                                                     Silicon         0.30-0.65                                                     Niobium         0.02-0.06                                                     Aluminum        0.03-0.06                                                     Sulfur          up to 0.02                                                    Phosphorus      up to 0.015                                                   Iron and incidental                                                           impurities      Balance;                                                      ______________________________________                                    

said steel substrate plate having an ASTM ferrite grain size of eight orfiner, a minimum yield point of 30 ksi, an ultimate tensile strength of60 ksi, and a minimum Charpy V-Notch energy of 50 foot-pounds at -50° F.in both the longitudinal and transverse directions, after having beensubjected to said thermal cycles.
 7. A low carbon steel plate inaccordance with claim 6 wherein the plate is at least 3/16" inthickness.